Transcriptional Activation of the fra-1 Gene by AP-1 Is Mediated by Regulatory Sequences in the First Intron

Abstract

Constitutive expression of c-Fos, FosB, Fra-1, or c-Jun in rat fibroblasts leads to up-regulation of the immediate-early gene fra-1. Using the posttranslational FosER induction system, we demonstrate that this AP-1-dependent stimulation of fra-1 expression is rapid, depends on a functional DNA-binding domain of FosER, and is a general phenomenon observed in different cell types. In vitro mutagenesis and functional analysis of the rat fra-1 gene in stably transfected Rat-1A-FosER fibroblasts indicated that basal and AP-1-regulated expression of the fra-1 gene depends on regulatory sequences in the first intron which comprise a consensus AP-1 site and two AP-1-like elements. We have also investigated the transactivating and transforming properties of the Fra-1 protein to address the significance of fra-1 up-regulation. The entire Fra-1 protein fused to the DNA-binding domain of Ga14 is shown to lack any transactivation function, and yet it possesses oncogenic potential, as overexpression of Fra-1 in established rat fibroblasts results in anchorage-independent growth in vitro and tumor development in athymic mice, fra-1 is therefore not only induced by members of the Fos family, but its gene product may also contribute to cellular transformation by these proteins. Together, these data identify fra-1 as a unique member of the fos gene family which is under positive control by AP-1 activity.

Transformation by ras modifies AP1 composition and activity

“…Expression of transdominant Jun or Fos mutants is capable of reverting transformed cells to a normal phenotype (Lloyd et al, 1991;Suzuki et al, 1994). Overexpression of wild-type Fra1 protein in NIH3T3 ®broblasts is by itself weakly transforming, though it is a more potent transforming gene in Rat1A cells (Bergers et al, 1995). Overexpression of cJun alone also leads to partial transformation of 3T3 ®broblasts.…”

“…Similar to c-jun, fra1 is known to be regulated by AP1 itself. Transcriptional activation of the fra1 gene by AP1 is mediated by regulatory sequences in the ®rst intron (Bergers et al, 1995). Elevated levels of fra1 mRNA were observed in ®broblasts overexpressing cJun, FosB, or Fra1 itself, indicating that the fra1 gene is up-regulated by itself or by other members of the AP1 family (Busslinger and Bergers, 1994;Miao and Curran, 1994).…”

Structure and chromosomal assignment of the mouse fra-1 gene, and its exclusion as a candidate gene for oc (osteosclerosis)

“…The DNA-binding speci®city of Fra-1/Jun heterodimers was found to be indistinguishable from that of c-Fos/Jun heterodimers using several AP-1 sites (Cohen et al, 1989). Fra-1 exhibits oncogenic potential similar to, but weaker than that of c-Fos, since overexpression in established rat ®broblasts leads to anchorage-independent growth and tumor development in athymic mice (Bergers et al, 1995). However, unlike c-Fos, Fra-1 lacks a transactivation domain and the entire Fra-1 protein fused to the DNA-binding domain of Gal4 was shown to lack any transcriptional activation function (Suzuki et al, 1991;Wisdom and Verma, 1993;Bergers et al, 1995).…”

“…Fra-1 exhibits oncogenic potential similar to, but weaker than that of c-Fos, since overexpression in established rat ®broblasts leads to anchorage-independent growth and tumor development in athymic mice (Bergers et al, 1995). However, unlike c-Fos, Fra-1 lacks a transactivation domain and the entire Fra-1 protein fused to the DNA-binding domain of Gal4 was shown to lack any transcriptional activation function (Suzuki et al, 1991;Wisdom and Verma, 1993;Bergers et al, 1995). Therefore, fra-1 can not only increase, but also repress total AP-1 activity depending on the status of the other Fos proteins in the cell (Suzuki et al, 1991;Groskopf and Linzer, 1994;Bergers et al, 1995;Welter et al, 1995;Yoshioka et al, 1995).…”

“…However, unlike c-Fos, Fra-1 lacks a transactivation domain and the entire Fra-1 protein fused to the DNA-binding domain of Gal4 was shown to lack any transcriptional activation function (Suzuki et al, 1991;Wisdom and Verma, 1993;Bergers et al, 1995). Therefore, fra-1 can not only increase, but also repress total AP-1 activity depending on the status of the other Fos proteins in the cell (Suzuki et al, 1991;Groskopf and Linzer, 1994;Bergers et al, 1995;Welter et al, 1995;Yoshioka et al, 1995). The transcriptional regulation of fra-1 is unique among members of the fos gene family, since only fra-1 is subject to positive control by AP-1 activity (BruÈ sselbach et al, 1995;Bergers et al, 1995).…”